Steven Earle

Chapter 5 Weathering and Soil

Introduction

Learning Objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

Explain why rocks formed at depth in the crust are susceptible to weathering at the surface

Describe the main processes of mechanical weathering, and the types of materials that are produced when mechanical weathering predominates

Describe the main processes of chemical weathering, and the products of chemical weathering of minerals such as feldspar, ferromagnesian silicates, and calcite

Explain the type of weathering processes that are likely to have taken place to produce a particular sediment deposit

Discuss the relationships between weathering and soil formation, and the origins of soil horizons and some of the different types of soil

Describe and explain the distribution of some of the important soil types in Canada

Explain the geological carbon cycle, and how variations in rates of weathering can lead to climate change

Figure 5.1 The Hoodoos, near Drumheller, Alberta, have formed from the differential weathering of sedimentary rock that was buried beneath other rock for close to 100 Ma [SE photo]

Weathering is what takes place when a body of rock is exposed to the “weather” — in other words, to the forces and conditions that exist at Earth’s surface. With the exception of volcanic rocks and some sedimentary rocks, most rocks are formed at some depth within the crust. There they experience relatively constant temperature, high pressure, no contact with the atmosphere, and little or no moving water. Once a rock is exposed at the surface, which is what happens when the overlying rock is eroded away, conditions change dramatically. Temperatures vary widely, there is much less pressure, oxygen and other gases are plentiful, and in most climates, water is abundant (Figure 5.1).

Weathering includes two main processes that are quite different. One is the mechanical breakdown of rock into smaller fragments, and the other is the chemical change of the minerals within the rock to forms that are stable in the surface environment. Mechanical weathering provides fresh surfaces for attack by chemical processes, and chemical weathering weakens the rock so that it is more susceptible to mechanical weathering. Together, these processes create two very important products, one being the sedimentary clasts and ions in solution that can eventually become sedimentary rock, and the other being the soil that is necessary for our existence on Earth.

The various processes related to uplift and weathering are summarized in the rock cycle in Figure 5.2.

Figure 5.2 Weathering can take place once a rock is exposed at surface by uplift and the removal of the overlying rock. [SE]

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Cover image: Mount Robson, British Columbia (3954 m, highest peak in the Canadian Rockies), with the Berg Glacier (left), the Mist Glacier (right) and Berg Lake in the foreground. Mount Robson is almost entirely made up of Cambrian sedimentary rock (ca. 500 Ma) that was pushed eastward and thrust upward during the formation of the Rocky Mountains, mostly during the past 100 million years by Heather Earle is CC BY.